Cantilevered Davits - Engineering Problem

Discussion in 'Boat Design' started by oceannavigator2, Mar 22, 2014.

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  1. rxcomposite
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    rxcomposite Senior Member

    It will deflect! Unless you align the width to vertical, that is the 2" width is in the vertical position.:)
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed. But that is the easy part.

    Without seeing the construction plan of the vessel to ascertain any localised fixing, its strength its quality and also load paths if any etc is simply impossible to say which would be best. The possible fixing locations may or may not affect the choice of section/material. But in the absence of such data, impossible to say.

    Which is one reason why using just numbers from a simple statics diagram based upon data which may or may not be correct only gives half the story..or a quarter of the story, more likely..
  3. oceannavigator2

    oceannavigator2 Previous Member

    Ok, but I really think this is a case of paralysis by analysis. Every day, davits to hold these same tenders are bought and simply bolted onto boats, through the grp with backing plates.

    Here is a set...

    The company (Edson) that designs and sells these does not need load paths of the vessel, nor construction plans, nor do they even have any idea what boat the davits will be used on. Yet. they are a very well known, respected multinational manufacturer of equipment life and limb depend on with full legal liability in the open market.

    They sell these, as do other manufacturers, they hold tenders up just fine and nothing goes wrong.

    You really do not need to know the laminate schedule of my aft main box beam to come up with davit designs. All you need is the correct material data, a plan (my original or Barry's superior design) and a diagram like Daquiri had drawn up.

    There is no reason for the level of design information you are posting about here... ??
  4. Ad Hoc
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    Ad Hoc Naval Architect

    That is simply down to the different working environments that one is involved in. Mine, is very clearly different from many posting on here.

    The risk mitigation is yours alone.
  5. oceannavigator2

    oceannavigator2 Previous Member

    very true.

    The point I was trying to make is that the glass and epoxy I use (Silvertip) creates some very, very stiff structures. And... that as you increase the composite scantlings, the beam will act as steel at some point.

    Hamg a liter of water off my main box beam that holds the hulls together. No deflection. Zero. So... davits could be made the same.

    This is, I think, where Groper was going with it.

    I have to admit, Barry's idea was far more elegant than my own. Will probably go with that.
  6. oceannavigator2

    oceannavigator2 Previous Member

    ok. understood. point taken. the object you are concerned with, the main box beam of a 50'+ catamaran, holds the entire boat together. It can handle a little RIB. ;)

    Ok, i guess this is put to rest.

    I will need to decide between aluminum and composite, using Barry's more elegant solution. From what i have learned here, aluminum will beat composite in compression, so I might as well go with aluminum posts with built in davits. The upside down "L". For good measure, i could bury these into the aft beam and backplate them as well, transfering most of the consequence of the tender right into the aft beam, like traditional, lower davits.

    Thank you for the thoughtful comments.
  7. rxcomposite
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    rxcomposite Senior Member

    It is only 1 meter long cantelever and you have lots of material around. Build one for destruction testing at 156 kg. Say a box beam of 120mm deep x 60mm wide. sidewalls of 4mm biax, upper caps of uni say 6mm top, 7-8mm bottom wrapped with at least 2 layers of biax. all in hand laid up method but squeege out as much resin as you can.

    For safety, hang a drum and slowly fill it up with water until you reach 156 liters add more to destruction, Note deflection as you fill it up.
  8. Rastapop
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    Rastapop Naval Architect

    There is no disagreement ;)
    There are many extremely knowledgeable people in these forums, and while I am completing my second engineering degree so am able to comment here with some accuracy, I don't pretend for one second pretend to be one of them.

    You'll notice that no-one here has contradicted what I said: you CAN get the same strength for less weight using aluminium or composites instead of steel.
    No-one here will argue with that.
  9. oceannavigator2

    oceannavigator2 Previous Member

    That sounds reasonable. I'll do it. Plenty of empty drums from resin around.

    Odd question, but I'd like to get this right: My main box beam is made from several complex layers of uni and triaxial. These are simply taped together with biax. That surprised me. Is this the same? Do I make the 4 panels of the box and biax tape them, or do I need to carry the material around the corner of the box?

    Also, when making the real upside down "L", do I need to do something special at the 90 degree point where all the stress is? A doubler maybe? In the shape of a triangle to help with that high stress area?


    And finally, the long part of the upside down L.... that's pretty much in compression, except when mainsheet loading directly above gives a tug. So... this post part should be made with ply doubler walls? Like a plywood stress form mast in many ways and buried in tha aft beam with a flange to take up all these loads?
  10. groper
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    groper Senior Member

    Nope... the vertical posts are in bending not compression for the L shaped davit. And yes, the 90deg corner has the highest stress. A 45deg bracket helps a lot.

    And no that was not where I was "going with it" earlier... read through carefully and you may see clearer, I can't put it any clearer than I already have...
  11. oceannavigator2

    oceannavigator2 Previous Member

    Ah, right! It used to be a post in compression and that changed with Barry's idea. Forgot about that. It will get tension and bending at 90 degrees to the tender bend load from the mainsheet above these posts.

    Will read bach again, but not sure what comment I have made that you are referring to about "going with it. " It is early morning here, so... :D
  12. Barry
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    Barry Senior Member

    The posts carry bending, shear and compression loads

    The pair will support half the load of the roof weight in compression.
    The pair will support the weight of the RIB in compression
    The pair will carry the weight of the RIB hung out on the arm say 40 inches to produce a bending moment which will produce compression and tension and shear FLOW in various parts of the post
    There will be a shear across the posts due to the moment at the lower post bracket

    But bending stresses are really tension on one side and compression on the other with a maximum shear flow stress down the neutral axis.

    So to totally analyze this you would take an infinitely small cube of material ( on paper) at the outermost edge of the bow side (then the stern side) of the post. You then calculate all the stresses and directions on this cube, develop a Mohr circles, graph it and see where the maximum stress is.
    This was complicated 40 years ago but now I suspect that there is a program that you can just insert the numbers into and get the location and the magnitude of the maximum stress

    I expect that Daq and Adhoc could do this as they seem to have a good understanding of the numbers and probably their income and reputation depends on doing the process correctly

    Ocean, you mentioned that you did not think that you needed a large factor of safety.

    Despite doing the calculations for static loading above which will get you close there are still other loads that WILL occur on the davits when underway that are difficult to calculate with 100% accuracy. Some but not all will be the dynamic forces due to the pitching of the RIB which will impart various shear, tension, compression and bending stresses into the members. Some of these will be a torque of the post. How the ropes are tied onto the RIB, ie perfectly vertical or at an angle off vertical. I suspect their are others.

    Airplanes which are acutely engineered, 100's of millions of dollars of engineering cost, using a factor of safety of 1.5 above maximum estimated loading still have the odd engine fall off the wing or other structural failure.

    I am not familiar with the box beam of the cat and would be concerned about "burying the posts" in them.
  13. oceannavigator2

    oceannavigator2 Previous Member

    Barry, thank you again. Very good input.

    I will put everyone's mind at ease about what ban aft box beam does in a catamaran and the loads it is sized 5x greater than needed for. It holds the aft part of a catamaran together, keeps the hulls apart and takes the main sheet loads. It can handle davits and it is normal to put the davits into the aft box beam. This is what is normally done.

    These are carbon davits buried in the box beam. Daiquiri put a picture up very early on of steel ones buried.

    My beam is twice the width of this one pictured. There are several bulkheads inside the beam.


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  14. Ad Hoc
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    Ad Hoc Naval Architect

    I suspect our real world approach to such a problem would be poo-poohed by those that do not consider such a conservative set of calculations is warranted. Seems there are many in this thread.

  15. rwatson
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    rwatson Senior Member

    Ummm - the davits can be galvanised AFTER welding. This would apply to the insides as well

    Stainless is a good solution, and if you can get the right alloy, with a good welder, would look the nicest.
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